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We explored the speciation and kinetics of the Pu(VI)-citrate and Pu(III)-citrate systems (pHm = 2.5-11.0, I = 0.1 M NaCl, T = 23 °C, O2(g) < 2 ppm) using ultraviolet-visible-near-infrared (UV-vis-NIR) spectrophotometry, solvent extraction, and PHREEQC modeling. Formation constants were determined for PuO2(HcitH)(aq) (log K°1,1 = 1.09 ± 0.05) and PuO2(HcitH)(citH)3- (log K°1,2 = -0.20 ± 0.07), and evidence for (PuO2)m(citH-k)n(OH)x2m(3+k)n-x was identified under alkaline conditions. Pu(VI) species were found to be less stable in the presence of citrate than in the absence of citrate (t ≤ 168 days); the rate of reduction increased with increasing pH. The direct reduction of Pu(VI) to Pu(IV) was required to fit experimental data in the presence of citrate but did not improve the fit for Pu in the absence of citrate. We also observed increased Pu(III) stability in the presence of citrate (t ≤ 293 days), with higher concentrations of Pu(III) favored at lower pH. Finally, we provide evidence of a radiolysis-driven mechanism for the citrate-mediated reduction of plutonium that involves electron transfer from the oxidative breakdown of citrate. Our work highlights the need to investigate the redox effect of organic ligands on plutonium oxidation states under repository-relevant conditions.

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A systematic study was conducted to investigate the effect of major groundwater ions (i.e., Ca2+, Na+, and HCO3-) on removal of hexavalent chromium (Cr(VI)) by an Fe(II)-phosphate mineral (i.e., vivianite). The batch experiments revealed that the second-order rate constant for Cr(VI) removal by vivianite with Ca2+ + CO32- (0.076-1.90 mM) and Na+ + HCO3- (0.26-6.50 mM) was 1.5-5.2 times lower than that without these ions. The removal kinetics of Cr(VI) by vivianite was abruptly slowed down with the increased ion concentration, which showed their inhibitory effect on the reaction. The results of the geochemical modeling and density functional theory calculations showed that the presence of Ca2+ + HCO3- and Na+ + HCO3- can form less favorable Cr(VI) species (i.e., CaCrO4(aq) and NaCrO4-) on the Fe-B site of vivianite surface, leading to the inhibitory effect observed in this study. Finally, the X-ray absorption spectroscopy results showed that reductive immobilization of Cr(VI) to Cr(III) occurred by structural Fe(II) oxidation of vivianite to amorphous mixed-valence Fe-phosphate via an inner-sphere complexation. The results suggest that the presence of Ca2+, Na+, and HCO3- in phosphorous-enriched iron-reducing environments may lower the remedial efficiency of Cr(VI) removal.

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Experimental and theoretical studies were conducted to identify the molecular-scale reaction mechanism for Cr(VI) removal by a ferrous phosphate mineral, vivianite. The surface-normalized rate constant for Cr(VI) removal in a vivianite suspension at pH 7 was higher than those obtained for other Fe(II)-containing minerals (i.e., magnetite and pyrite). The highest rate constant was obtained at pH 5, which was 35- and 264-times higher than those obtained at pH 7 and 9, respectively, indicating the dramatic acceleration of removal kinetics with decreasing pH of suspension. The X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) spectroscopy revealed that Cr(VI) removal involved reduction of Cr(VI) to Cr(III) coupled with oxidation of Fe(II) to Fe(III) on the vivianite surface. In addition, the density functional theory (DFT)-optimized structure of the Cr(VI)-vivianite complex was consistent with that obtained from extended X-ray absorption fine structure (EXAFS) spectroscopy and revealed the transformation of vivianite to amorphous Fe(III) phosphate. We also demonstrated that both Cr(VI) species, HCrO4̅ and CrO42-, can effectively bind to the vivianite surface, particularly on the Fe sites having 6 neighboring Fe molecules with 4 H2O and 2 PO4 moieties. Our results show that Cr(VI) is readily reduced to Cr(III) by vivianite via adsorption and inner-sphere complexation, suggesting that in anoxic iron-phosphate-enriched environments, vivianite may significantly influence the fate and transport of Cr(VI).

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Plutonium plays an important role within nuclear waste materials because of its long half-life and high radiotoxicity. The aim of this study was to investigate with high spatial resolution the reactivity of the more oxidized forms of Pu(V,VI) within Opalinus Clay (OPA) rock, a heterogeneous, natural argillaceous rock considered as a potential repository host. A combination of synchrotron based X-ray microprobe and bulk techniques was used to study the spatial distribution and molecular speciation of Pu within OPA after diffusion and sorption processes. Microscopic chemical images revealed a pronounced impact of geochemical heterogeneities concerning the reactivity of the natural barrier material. Spatially resolved X-ray absorption spectroscopy documented a reduction of the highly soluble Pu(V,VI) to the less mobile Pu(IV) within the argillaceous rock material, while bulk investigations showed second-shell scattering contributions, indicating an inner-sphere sorption of Pu on OPA components. Microdiffraction imaging identified the clay mineral kaolinite to play a key role in the immobilization of the reduced Pu. The findings provide strong evidence that reduction and immobilization do not occur as linked processes on a single reactive phase but as decoupled, subsequent, and spatially separated reactions involving different phases of the OPA.

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The pH dependence (1-7) of Am(III) complexation with lactate in aqueous solution is studied using extended X-ray absorption fine-structure (EXAFS) spectroscopy. Structural data (coordination numbers, Am--O and Am--C distances) of the formed Am(III)-lactate species are determined from the raw k(3)-weighted Am LIII-edge EXAFS spectra. Between pH 1 and pH 6, Am(III) speciation shifts continuously towards complexed species with increasing pH. At higher pH, the amount of complexed species decreases due to formation of hydroxo species. The coordination numbers and distances (3.41-3.43 Å) of the coordinating carbon atoms clearly point out that lactate is bound `side-on' to Am(III) through both the carboxylic and the α-hydroxy function of lactate. The experimentally determined coordination numbers are compared with speciation calculations on the basis of tabulated thermodynamic stability constants. Both EXAFS data and thermodynamic modelling are in very good agreement. The EXAFS spectra are also analyzed by iterative transformation factor analysis to further verify the determined Am(III) speciation and the used structural model.

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Capillary electrophoresis (CE) was used to separate the neptunium oxidation states Np(IV) and Np(V), which are the only oxidation states of Np that are stable under environmental conditions. The CE setup was coupled to an inductively coupled plasma mass spectrometer (Agilent 7500ce) using a Mira Mist CE nebulizer and a Scott-type spray chamber. The combination of the separation capacity of CE with the detection sensitivity of inductively coupled plasma mass spectrometry (ICP-MS) allows identification and quantification of Np(IV) and Np(V) at the trace levels expected in the far field of a nuclear waste repository. Limits of detection of 1 × 10(-9) and 5 × 10(-10) mol L(-1) for Np(IV) and Np(V), respectively, were achieved, with a linear range from 10(-9) to 10(-6) mol L(-1). The method was applied to study the redox speciation of the Np remaining in solution after interaction of 5 × 10(-7) mol L(-1) Np(V) with Opalinus Clay. Under mildly oxidizing conditions, a Np sorption of 31% was found, with all the Np remaining in solution being Np(V). A second sorption experiment performed in the presence of Fe(2+) led to complete sorption of the Np onto the clay. After desorption with HClO(4), a mixture of Np(IV) and Np(V) was found in solution by CE-ICP-MS, indicating that some of the sorbed Np had been reduced to Np(IV) by Fe(2+).

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Synchrotron-based X-ray absorption spectroscopy has been used to determine the chemical speciation of Np sorbed on Opalinus Clay (OPA, Mont Terri, Switzerland), a natural argillaceous rock revealing a micro-scale heterogeneity. Different sorption and diffusion samples with Np(V) were prepared for spatially resolved molecular-level investigations. Thin sections of OPA contacted with Np(V) solution under aerobic and anaerobic conditions as well as a diffusion sample were analysed spatially resolved. Micro-X-ray fluorescence (µ-XRF) mapping has been used to determine the elemental distributions of Np, Fe and Ca. Regions of high Np concentration were subsequently investigated by micro-X-ray absorption fine structure spectroscopy to determine the oxidation state of Np. Further, micro-X-ray diffraction (µ-XRD) was employed to gain knowledge about reactive crystalline mineral phases in the vicinity of Np enrichments. One thin section was also analysed by electron microprobe to determine the elemental distributions of the lighter elements (especially Si and Al), which represent the main elements of OPA. The results show that in most samples, Np spots with considerable amounts of Np(IV) could be found even when the experiments were carried out in air. In some cases, almost pure Np(IV) L(III)-edge X-ray absorption near-edge structure spectra were recorded. In the case of the anaerobic sample, the µ-XRF mapping showed a clear correlation between Np and Fe, indicating that the reduction of Np(V) is caused by an iron(II)-containing mineral which could be identified by µ-XRD as pyrite. These spatially resolved investigations were complemented by extended X-ray absorption fine structure measurements of powder samples from batch experiments under aerobic and anaerobic conditions to determine the structural parameters of the near-neighbour environment of sorbed Np.